The present invention relates generally, as indicated, to a pulse width modulator digital servo system and, more particularly, to such a system that combines the high resolution of an analog input with the advantages of convenient data manipulation and monitoring, noise immunity, and the like of a digital system for accurate output control function usually without any additional analog to digital (A/D) or digital to analog (D/A) converters.
One type of analog transducer that may be conveniently utilized with the invention is a linear variable phase transformer (LVPT) that may be used with a digital phase detector to produce positional information in a digital format, for example, as described in copending U.S. patent application Ser. No. 758,654, filed Jan. 12, 1977, for "Linear Variable Phase Transformer", which application is assigned to the same assignee as the present application.
Conventional analog transducers usually include potentiometers, which may directly insert inaccuracies into output information, and linear variable differential transformers (LVDT), which require costly A/D converters to produce digital outputs to facilitate interfacing with conventional digital equipment, etc. These converters, of course, increase the cost of a servo system.
The LVPT employed in the present invention is a phase modulator and, as such, has substantially infinite resolution in providing an absolute, time-based response. The ultimate resolution of the output information in accordance with the digital system of the invention, as will be described below, is limited only by the ratio of the digital clock frequency and the LVPT excitation frequency. In contrast, typical analog transducers employed in a servo system or the like have their digital resolution limited by the A/D converter by which they are interfaced to the servo system or other digital system. Moreover, conventionally to increase from 8 bit resolution to 12 bit resolution may be relatively expensive in A/D converters, but is appreciably less expensive in the phase modulated system of the present invention. Additionally, the phase modulated system of the invention offers higher immunity to noise and other types of interference.
Two types of direct digital encoders, namely, the absolute and the incremental digital encoders, are available that do not require A/D conversion. However, to obtain a high degree of resolution in an absolute digital encoder normally requires an undesirably extremely large number of parallel lines to the electronics; and although incremental digital encoders offer simpler cabling, a small amount of noise often will invalidate the output information therefrom.